The present invention relates to a constant current output driver, for example, a constant current output driver for emitting a light from a light emitting element such as an organic EL (electro luminescent) panel.
A light emitting diode (LED) and an organic EL (also called xe2x80x9corganic LEDxe2x80x9d) has widely been spread as an image source in a display unit such as a direct-view display unit or a virtual image display unit since a high visibility is obtained because the light emitting diode and the organic EL per se emit a relatively large amount of high-luminance light, and a display speed is higher than that of a TFT (thin film transistor) liquid crystal or the like to make it difficult to produce a latent image. Since the display panel using those LED or organic EL is of the current driven type, a constant current output driver is employed in the drive device.
FIG. 22 shows the structure of a conventional display device using the organic EL of this type.
As shown in FIG. 22, the organic EL display unit includes an organic EL panel 1, a scanning circuit 2, a drive circuit 3 and a not-shown light emission control circuit that controls the switching operation of the scanning circuit 2 and the drive circuit 3.
The organic EL panel 1 includes anode lines A1 to Am and cathode lines B1 to Bn which are arranged in a simple matrix (lattice), and organic EL elements E11 to Emn connected to the respective intersecting positions of the anode lines and the cathode lines which are arranged in the form of a lattice. The cathode lines B1 to Bn are connected to a scanning circuit 51, and the anode lines A1 to Am are connected to the drive circuit 3.
The scanning circuit 2 includes switches S21 to S2n and conducts scanning operation by sequentially setting the cathode lines B1 to Bn to the earth potential (0 V) while those switches S21 to S2n sequentially change over to the earth terminal side at given time intervals.
The drive circuit 3 includes switches S31 to S3m and constant current elements C31 to C2m connected to a power supply VDD. The drive circuit 3 is so designed as to connect the anode lines A1 to Am to the constant current elements C31 to C3m while controlling the on/off operation of the respective switches S31 to S3m in synchronism with the scanning operation of the cathode line scanning circuit 2 and to supply a drive current to the organic EL at a desired intersecting position, to thereby emit a light.
A case in which the cathode lines are scanned and the anode lines are driven is shown in FIG. 22. The same effect is obtained in the structure in which the anode lines are scanned and the cathode lines are driven.
Now, it is assumed that the switch S22 in the scanning circuit 2 is connected to the earth side to scan a row of the cathode line B2. At this timing, when the switches S31 and S33 in the drive circuit 3 are changed over to the constant current elements C31 and C33 side (turn on), currents I12 and I13 flow in the light emitting elements E12 and E32 to emit a light.
An inverse bias voltage VCC identical in potential with the supply voltage is applied to other cathode lines B1 and B3 to Bn other than the cathode line B2 which is being scanned, to thereby prevent an error in light emission.
The organic EL is made to emit a light at an arbitrary position by repeating the scanning of the cathode lines B1 to Bn and the driving of the anode lines A1 to Am at a high speed, and display is made so that the respective organic ELs emit a light on the entire screen at the same time.
In this situation, in order to prevent the amount of light emission from being made ununiform due to a difference between the current values that flow in the respective organic ELs which is caused by a difference in the wiring distance between the organic ELs, the respective anode lines A1 to Am are connected to the constant current elements C31 to C3m that function as the constant current source through the switches S31 to S3m. 
FIG. 23 shows the unknown structure proposed in a case where the conventional constant current output driver made up of the constant current elements of the above type and the switches that connect and disconnect the constant current elements and the anode lines is formed of the enhancement MOSFET (metal oxide field effect transistor).
As shown in FIG. 23, the constant current output driver is designed such that a p-channel MOSFET 4 that functions as the constant current element, and a p-channel MOSFET 5 and an n-channel MOSFET 6 which function as the switching elements are connected in series. The supply voltage VDD is applied to the drain of the MOSFET 4, and a voltage VGC which is always constant is applied to the gate of the MOSFET 4 in order to output a constant current.
An input terminal IN is connected to both the gates of the MOSFETs 5 and 6, an output terminal OUT is connected to the source of the MOSFET 5 and the drain of the MOSFET 6, and the anode lines of the organic EL panel are connected to the output terminal OUT.
In the constant current output driver made up of the MOSFETs as described above, when a switching signal which is supplied to the input terminal changes from on (high level) to off (low level), the MOSFET 6 changes from on to off and the MOSFET 5 changes from off to on. As a result, the output terminal OUT which has been connected to the ground is connected to the MOSFET 4 which functions as the constant current element through the MOSFET 5 to output a constant current.
FIG. 25 shows the structure of a display unit using organic ELS for color display.
As shown in FIG. 25, in an organic EL panel 1 for color display, three times as many anode lines A1 to As (s=3m) as in a monochrome display are disposed, and anode terminals Al to As for connection to the drive circuit are disposed.
In the organic EL panel 1 for color display, organic ELs for red (R) are connected at the intersecting positions of anode lines A1, A4, . . . Asxe2x88x922 and the respective cathode lines B1 to Bn, organic ELs for green (G) are connected at the intersecting positions of anode lines A2, A5, . . . Asxe2x88x921 and the respective cathode lines B1 to Bn, and organic ELs for red (R) are connected at the intersecting positions of anode lines A3, A6, . . . As and the respective cathode lines B1 to Bn, as in FIG. 22 although being not shown.
Those light emitting elements for R, G and B are different in optimum constant current value, respectively, when emitting the light. For that reason, as shown in FIG. 25, in the conventional drive circuit that drives the organic EL panel 1 for color display, there are used a drive circuit 3R for R, a drive circuit 3G for G and a drive circuit 3B for B. Each of the drive circuits 3R, 3G and 3B includes m (s/3) constant current output sections and m constant current output terminals O1 to Om.
As shown in FIG. 25, in order to drive the organic ELs for R which are disposed at every three organic ELs, the respective output terminals O1 to Om of the drive circuit 3R for R are connected with the anode terminals A1, A4, . . . Asxe2x88x922 of the organic EL panel 1 for color display. Likewise, the respective output terminals O1 to Om of the drive circuit 3GR for G are connected with the anode terminals A2, A5, . . . Asxe2x88x9212 of the organic EL panel 1 for color display, and the respective output terminals O1 to Om of the drive circuit 3B for B are connected with the anode terminals A3, A6, . . . As.
As described above, in the case where the conventional constant current output driver is formed of MOSFETs as. shown in FIG. 23, the current flowing in the respective organic EL elements can be made substantially constant by use of the constant current element.
However, the conventional constant current output driver suffers from the following problems.
A first problem is that over-shoot occurs in the current outputted from the output terminal OUT of the conventional respective constant current output drivers, and there actually exists a moment where a current equal to or more than a given current flows in the light emitting element.
FIG. 24 shows the simulation result of an output current in the constant current output driver shown in FIG. 23. As shown in FIG. 24, a large current indicated by an arrow A flows as soon as the constant current output turns on, and a large current indicated by an arrow B flows as soon as the constant current output turns off.
The current indicated by the arrow A is that charges charged in the source of the MOSFET 5 while the MOSFET 5 shown in FIG. 23 is off flows in the output terminal as soon as the MOSFET 5 turns on. At the same time, the current flows in the minus direction but is canceled by the above current and does not appear in the figure. This current in the minus direction is that a high frequency component flows through the capacitor of a gate insulating film because the gate voltage IN of the MOSFET 5 and the MOSFET 6 changes from VDD to the ground.
The current indicated by the arrow B is that a high frequency component flows through the capacitor of the gate insulating film because the gate voltage IN of the MOSFET 5 and the MOSFET 6 changes from the ground to VDD.
The current in the minus direction indicated by an arrow C is a current resulting from discharging the charges stored in the capacity component of the organic EL connected to the output terminal OUT to the ground, which is not abnormal.
A second problem is stated below.
In the case of driving the organic EL panel, because a wiring resistance is large, the wiring resistance is largely different because of a difference in the wiring length due to the. position of the organic EL elements on the organic EL panel 1 shown in FIG. 22. Therefore, a voltage applied to the respective constant current elements is also largely different. As a result, a demand exits for the constant current element of the drive circuit 3 that the voltage dependency change of the output current value is very small. However, in the case where the conventional constant current output driver is formed of MOSFETs as shown in FIG. 23, in order to reduce the voltage dependency change of the output constant current value, it is necessary to increase the gate length of the p-channel MOSFET 4 that functions as the constant current element with a problem that the IC chip size of the drive circuit 3 increases.
A third problem is that if the MOSFET is employed as the constant current element of the constant current output driver as shown in FIG. 23, the switching noise adversely affects the gate potential of the MOSFET 4 that functions as the constant current element, and the constant current value fluctuates. In the case where the organic EL panel using a plurality of drivers is driven, because many noises are received at various timings, a larger problem is caused.
A fourth problem is stated below.
The MOSFET has a temperature characteristic that the output current value changes in accordance with the temperature change. Also, the current value outputted from the constant current FET 50 is nearly proportional to the square of the gate voltage.
For that reason, if a constant voltage VGC is always applied to the gate G of the constant current FET 50 regardless of the temperature change, there arises such a problem that the output current largely changes in accordance with the temperature change.
A fifth problem is stated below.
In the conventional constant current output driver in the case where color display is conducted using the organic ELs, the m constant current output sections that drive the light emitting elements for R are disposed in the drive circuit 3R for R together, the m constant current output sections that drive the light emitting elements for G are disposed in the drive circuit 3R for G together, and the m constant current output sections that drive the light emitting elements for B are disposed in the drive circuit 3R for G together. On the other hand, the respective light emitting elements of the organic EL panel 1 for color display and their anode terminals A1 to As are disposed in the order of R, G and B.
For that reason, in the case where the drive circuit 3R for R, the drive circuit 3G for G and the drive circuit 3B for B are connected to the organic EL panel 1 for color display, because the wirings intersects with each other as shown in FIG. 25, multi-layer wirings must be provided, to thereby increase the costs.
Also, in the case where the organic EL panel 1 for color display is mounted on a COG (chip on glass), because it is extremely difficult to provide the multi-layer wirings, the display device could not be substantially mounted on the COG with the structure shown in FIG. 25.
The present invention has been made in order to solve the above-described problems, and therefore an object of the present invention is to provide a constant current output driver which reduces the occurrence of over-shoot at the first, which is extremely small in the voltage dependency change of the output constant current value at the second, which reduces the adverse affect of the switching noise on the constant current property at the third, which reduces a change in the output current value due to the temperature change at the fourth, and which can be connected to the light emitting element panel for color display through single-layer wirings.
In the present invention as recited in claim 1, as conceptually shown in FIG. 1, the above object is achieved by the provision of a constant current output driver for emitting a light from a light emitting element by supplying a constant current, the constant current output driver comprising: a constant current output element that outputs a constant current (100); a first switching element (101) disposed between the constant current output element and a power supply (VDD), for electrically connecting and disconnecting the power supply and the constant current output element; and a first output terminal (102) connected to a current output side of the constant current element with no switching element disposed therebetween.
In the present invention as recited in claim 2, as conceptually shown in FIG. 2, the above object is achieved by the provision of a constant current output driver for emitting alight from a light emitting element by supplying a constant current, the constant current output driver comprising: a constant current output element (100) that outputs a constant current; a first switching element (101) disposed between the constant current output element and a power supply (VDD), for electrically connecting and disconnecting the power supply and the constant current output element; a second switching element (103) connected between current output side of the constant current output element and a second terminal (104), for performing an on/off operation in association with the on/off operation of the first switching element; and a first output terminal (102) connected between the current output side of the constant current output element and the second switching element (103) with no switching element therebetween.
In the present invention as recited in claim 3, as conceptually shown in FIG. 3, the above object is achieved by the provision of a constant current output driver comprising:
a first field effect transistor (50) for outputting a constant current with the application of a constant voltage (VGC) to a gate thereof;
a second field effect transistor (40) disposed between the first field effect transistor and a power supply (VDD), for electrically connecting and disconnecting the power supply and the first field effect transistor; and
A first output terminal (OUT) connected to a current output side of the first field effect transistor with no switching element therebetween.
In the present invention as recited in claim 4, as conceptually shown in FIG. 4, the above object is achieved by the provision of a constant current output driver comprising: a first field effect transistor (50) for outputting a constant current with the application of a constant voltage (VGC) to a gate thereof; a second field effect transistor (40) disposed between the first field effect transistor and a power supply (VDD), for electrically connecting and disconnecting the power supply and the first field effect transistor; a third field effect transistor (70) connected in series with a current output side of the first field effect transistor and having a gate thereof applied with a constant voltage (VGC2) different from the constant voltage which is applied to the gate of the first field effect transistor; and a first output terminal (OUT) connected to a current output side of the third field effect transistor through no switching element.
In the present invention as recited in claim 5, as conceptually shown in FIG. 5, the above object is achieved by the provision of a constant current output driver comprising: a first field effect transistor (50) for outputting a constant current with the application of a constant voltage (VGC) to a gate thereof; a second field effect transistor (40) disposed between the first field effect transistor and a power supply (VDD), for electrically connecting and disconnecting the power supply and the first field effect transistor; a third field effect transistor (60) forming a channel different from that of the second field effect transistor, connected between a current output side of the first field effect transistor and a second terminal (a terminal to which the ground is connected), and having a gate thereof commonly connected to an input terminal (IN) together with a gate of the second field effect transistor; and a first output terminal (OUT) connected between a current output side of the first field effect transistor and the third field effect transistor through no switching element.
In the present invention as recited in claim 6, as conceptually shown in FIG. 7, the above object is achieved by the provision of a constant current output driver comprising: a first field effect transistor (50) for outputting a constant current with the application of a constant voltage (VGC) to a gate thereof; a second field effect transistor (40) disposed between the first field effect transistor and a power supply (VDD), for electrically connecting and disconnecting the power supply and the first field effect transistor; a third field effect transistor (70) connected in series with a current output side of the first field effect transistor and having a gate thereof applied with a constant voltage (VGC2) different from the constant voltage which is applied to the gate of the first field effect transistor; a fourth field effect transistor (60) forming a channel different from that of the second field effect transistor, connected between a current output side of the third field effect transistor and a second terminal (a terminal to which the ground is connected), and having a gate thereof commonly connected to an input terminal (IN) together with a gate of the second field effect transistor; and a first output terminal (OUT) connected between the current output side of the third field effect transistor and the fourth field effect transistor through no switching element.
In the present invention as recited in claim 7, as conceptually shown in FIG. 8, the above object is achieved by the provision of a constant current output driver comprising: a first field effect transistor (50) connected to a power supply (VDD), for outputting a constant current with the application of a constant voltage (VGC) to a gate thereof; a second field effect transistor (40) connected in series with a current output side of the first field effect transistor, for conducting switching operation; a third field effect transistor (70) connected in series with a current output side of the second field effect transistor and having a gate thereof applied with a constant voltage (VGC2) different from the constant voltage which is applied to the gate of the first field effect transistor; a fourth field effect transistor (60) forming a channel different from that of the second field effect transistor, connected between a current output side of the third field effect transistor and a second terminal, and having a gate thereof commonly connected to an input terminal (IN) together with a gate of the second field effect transistor; and a first output terminal (OUT) connected between the current output side of the third field effect transistor and the fourth field effect transistor through no switching element.
In the present invention as recited in claim 8, as conceptually shown in FIG. 9, the above object is achieved by the provision of a constant current output driver comprising: a first field effect transistor (50) connected to a power supply (VDD), for outputting a constant current with the application of a constant voltage (VGC) to a gate thereof; a second field effect transistor (70) connected in series with a current output side of the first field effect transistor and having a gate thereof applied with a constant voltage (VGC2) different from the constant voltage which is applied to the gate of the first field effect transistor; a third field effect transistor (40) connected in series with a current output side of the second field effect transistor for conducting switching operation; a fourth field effect transistor (60) forming a channel different from that of the third field effect transistor, connected between a current output side of the third field effect transistor and a second terminal, and having a gate thereof commonly connected to an input terminal (IN) together with a gate of the second field effect transistor; and a first output terminal (OUT) connected between the current output side of the third field effect transistor and the fourth field effect transistor through no switching element.
In the present invention as recited in claim 9, the above object is achieved by the provision of a constant current output driver comprising: an output terminal (OUT); a first field effect transistor (50) connected between the output terminal and a power supply (VDD), for outputting a constant current with the application of a constant voltage to a gate thereof; switching means (40) connected between the first field effect transistor and the output terminal or the power supply, for electrically connecting and disconnecting the first field effect transistor and the output terminal or the power supply; and a constant voltage applying circuit (31) having a temperature characteristic which changes in the same manner as the temperature characteristic of the first field effect transistor, for applying a constant voltage to the first field effect transistor at a constant temperature.
In the present invention as recited in claim 10, the above object is achieved by the provision of a constant current output driver in the invention of claim 9, the constant voltage applying circuit comprises: a voltage dividing means (R), a second field effect transistor (51) connected in serial between the resistor means and the power supply and having a gate thereof connected in a saturated state, wherein the gate of the second field effect transistor is connected to the gate of the first field effect transistor.
In the present invention as recited in claim 11, the above object is achieved by the provision of a constant current output driver in the invention of claim 10 wherein the first switching means comprises a third field effect transistor (40) having a gate which inputs a switching signal; and
wherein there is further provided a fourth field effect transistor (41) which has a gate connected to the ground and is connected to a power supply side or its opposite side of the second field effect transistor in association with a connection position of the first switching means.
In the present invention as recited in claim 12, the above object is achieved by the provision of a constant current output driver in the invention of claim 10 wherein the first field effect transistor and the second field effect transistor are formed of the same standard elements.
In the present invention as recited in claim 13, the above object is achieved by the provision of a constant current output driver in the invention of any one of claims 9 to 12 wherein the resistor means comprises a ladder resistor, a variable resistor, a terminal to which a resistor is externally attached, or a resistor and a terminal to which another resistor is externally attached in parallel to the terminal.
In the present invention as recited in claim 14, the above object is achieved by the provision of a constant current output driver in the invention of any one of claims 9 to 13 wherein a gate of the second field effect transistor is connected to an input of a voltage follower circuit (71), and an output of the voltage follower circuit is connected to a gate of the first field effect transistor.
In the present invention as recited in claim 15, the above fifth object is achieved by the provision of a constant current output driver wherein there are formed, in one integrated circuit, a plurality of output terminals; field effect transistors the number of which is equal to that of the output terminals, which are connected between the output terminals and a power supply and which output a constant current with the application of a given voltage to gates thereof; switching means for electrically connecting and disconnecting the respective field effect transistors and the respective output terminals or the. power supply, independently; a first constant current control section that generates a first voltage; a second constant current control section that generates a second voltage; a third constant current control section that generates a third voltage; a first wiring that connects an output of the first constant current control section to every three gates of the plurality of field effect transistors; a second wiring that connects an output of the second constant current control section to the gates of the respective field effect transistors adjacent to the respective field effect transistors to which the first wiring is connected; and a third wiring that connects an output of the third constant current control section to the gates of the respective field effect transistors further adjacent to the respective field effect transistors to which the second wiring is connected.
As described above, according to the present invention, for example, in the case where the constant current output driver is applied to an organic EL panel for color display, three kinds of field effect transistors that output different constant current values and an output terminal are disposed in correspondence with the arrangement order of three kinds of light emitting elements (the respective light emitting elements of R, G and B) which are sequentially connected to the respective anode terminals A1 to A2 arranged in a line. Accordingly, it is unnecessary to provide multi-layer wirings, and the wiring work is simplified. Also, since the multi-layer wirings is unnecessary, it is possible to use the constant current output driver for a display unit such as an organic EL for color display mounted on a COG.
Also, in the present invention as recited in claim 15, the number of the output terminals and the number of the field effect transistors are 192, respectively. In this way, 192 (a common multitude of 3xc3x978) pieces each of the output terminals and the field effect transistors are arranged. As a result, it is possible to make a chip size (for example, 20 mm) suitable for mounting, and the mounting cost can be reduced.
In the present invention as recited in claim 15, there are provided, in the claim 15, a first wiring terminal to which the first wiring is connected, a second wiring terminal to which the second wiring is connected, and a third wiring terminal to which the third wiring is connected. Since the respective first to third wiring terminals are connected in this way, it is possible to monitor the voltage outputted from the first to third constant voltage circuits.
In the present invention as recited in claim 18, there are provided two of the first constant current control sections, two of the second constant current control sections and two of the third constant current control sections, respectively; wherein the first wiring includes a first voltage wiring that connects both the outputs of two first constant current control sections to each other, and a first gate wiring that connects the first voltage wiring and the respective gates; wherein the second wiring includes a second voltage wiring that connects both the outputs of two second constant current control sections to each other, and a second gate wiring that connects the second voltage wiring and the respective gates; and wherein the third wiring includes a third voltage wiring that connects both the outputs of two third constant current control sections to each other, and a third gate wiring that connects the third voltage wiring and the respective gates.
Since there are provided two first constant current control section, two second constant current control section and two third constant current control section as described above, it is possible to adjust the inclination of a change in the voltage applied to the respective field effect transistors.
Also, in the present invention, a resistor is disposed on a voltage wiring between at least one pair of gate wirings. In this case, the voltage wiring can be formed of a polysilicon resistor.
Also, in the present invention as recited in claim 21, there are formed, in one integrated circuit, a plurality of output terminals; field effect transistors the number of which is equal to that of the output terminals, which are connected between the output terminals and a power supply and which output a constant current with the application of a given voltage to gates thereof; switching means for electrically connecting and disconnecting the respective field effect transistors and the respective output terminals or the power supply, independently; first, second and third voltage input terminals to which a voltage is applied; first, second and third voltage output terminals from which the voltage is outputted; a first voltage wiring that connects the first voltage input terminal and the first voltage output terminal; a second voltage wiring that connects the second voltage input terminal and the second voltage output terminal; a third voltage wiring that connects the third voltage input terminal and the third voltage output terminal; a first gate wiring that connects the first voltage wiring to every three gates of the plurality of field effect transistors; a second gate wiring that connects the second voltage wiring to the gates of the respective field effect transistors adjacent to the respective field effect transistors to which the first wiring is connected; and a third gate wiring that connects the third voltage wiring to the gates of the respective field effect transistors further adjacent to the respective field effect transistors to which the second wiring is connected.
As the piezoelectric input terminal to which a voltage is applied is provided as described above, the first voltage, the second voltage and the third voltage applied to the gates of the field effect transistors can be applied from the exterior of the constant current output driver. In addition, since the voltage output terminals from which the voltage is outputted are provided, it is possible to sequentially connect a plurality of constant current output drivers. Also, the first voltage, the second voltage and the third voltage can be applied from the voltage output terminal, and since there is provided a difference between the applied voltage from the voltage input terminal and the applied voltage from the voltage output terminal, it is possible to adjust the inclination of a change in the voltage applied to the respective field effect transistors.
In the present invention, there may be used the voltage supply device of the constant current output driver in which there is formed, in one integrated circuit, a first constant current control section that generates the first voltage, a second constant current control section that generates the second voltage, a third constant current control section that generates the third voltage, a first terminal to which the output of the first constant current control section is connected, a second terminal to which the output of the second constant current control section is connected, and a third terminal to which the output of the third constant current control section is connected, and the first, the second and the third terminals are connected to the first, the second and the third voltage input terminals to apply a voltage.